1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the semiconductor device, particularly to a semiconductor device with a high-voltage transistor and a low-voltage transistor and to a method for manufacturing the semiconductor device.
2. Related Art
Recently, with the advance of a shrinking of a semiconductor device, a low-viscosity material is used as an isolation insulating film buried between element regions. However, when the low-viscosity material is buried between element regions, a crystal defect is generated in the element region by a stress. The crystal defect causes junction leakage in semiconductor elements such as a MOS (Metal Oxide Semiconductor) transistor. The crystal defect causes a problem in not only an n-type low-voltage transistor but also a p-type low-voltage transistor.
It is well known that the crystal defect has a relationship with a depth of the isolation insulating film (see JP-A No. 2005-353892). Further, the crystal defect tends to cause a problem in an element region of a low-voltage transistor having the small element region, due to a gate length of the transistor.
However, many semiconductor devices having different film thicknesses of the gate insulating films are manufactured in nonvolatile memories. When the element region of the low-voltage transistor is lower than that of the isolation insulating film adjacent to an element region of a high-voltage transistor, the junction leakage of the low-voltage transistor is easy to generate due to the crystal defect.